Studies have been actively conducted on second-generation high temperature superconducting (HTS) coated conductors to form a thin buffer layer with a biaxially aligned textured structure or a superconducting film on a metal substrate. The second-generation HTS coated conductors have a superior current carrying capability per unit area than conventional metal wires. The second-generation HTS coated conductors can be applied in fields such as electrical power generation with minimal power loss, MRI, superconducting magnetic levitation trains, superconducting propulsion ships, and so on.
The second-generation HTS coated conductors can be fabricated by metal-organic deposition (MOD). For this purpose, a REBa2Cu3O7-xbased (RE=rare earth element such as Y, La, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, etc. or any combination thereof) superconducting film is uniformly coated on a film with a biaxially aligned textured structure and subjected to heat treatment for superconductor synthesis. In order to form an epitaxial superconducting film, a metal-organic precursor solution including superconducting metal ions is needed.
Conventionally, a metal-organic precursor solution showing the best properties required a trifluoroacetate (TFA) solution. According to the conventional method, the precursor solution was prepared by dissolving yttrium (Y)-acetate, barium (Ba)-acetate, and copper (Cu)-acetate into a TFA solution in accordance with cation ratios of a final superconducting product (for example, Y:Ba:Cu=1:2:3), respectively, and then, through a vaporizing distillation process and a remelting-polymerization (refluxing) process, a precursor solution, in which cation ratios of Y, Ba, Cu is 1:2:3, was fabricated. However, when a calcination process is performed by using a precursor solution prepared through the conventional method, a large amount of HF gas produced by reaction of fluorine contained in the precursor with water vapor may be released. The HF gas can be responsible for crack generation when a thick film (about 0.2 μm thick or more) is fabricated. It has been a challenging problem to produce MOD-processed second-generation HTS coated conductors with high critical currents. Therefore, it is impossible to obtain a thick film (about 1 μm thick or more) with only one-time coating process by the conventional method. In order to maximally inhibit cracking generation during fabrication of a thick film (about 1 μm thick or more), a prolonged time for a calcination process is needed, and several applications are required.